The evolved UTRAN (E-UTRAN) supports for downlink transmissions both single-user (SU) and multi-user (MU) MIMO techniques, which makes it necessary to support switching between said modes.
In a single-user (SU-)MIMO transmission scheme all MIMO streams are assigned to a single user at a time allowing this user to achieve very high peak data rate. This approach is feasible when the base station has buffered a sufficient amount of data traffic to be transmitted to a user and all MIMO streams exhibit sufficiently good channel quality. Typically, single-user MIMO provides higher gains in less dispersive channel environments.
In a multi-user (MU-)MIMO transmission scheme several UEs are assigned the same resource block(s) on different MIMO streams at a time. This scheme is more useful for a large number of simultaneously active users in the system when these users do not require very high peak data rates. The obvious solution is to share the downlink resources among these active users.
Reference signals or pilot signals are used for cell search and acquisition, cell identification, UE measurement and channel estimation. There are two types of reference signals: Common reference signal and dedicated reference signal.
In FDD-based E-UTRAN system it is assumed that UEs in either single-user MIMO (SU-MIMO) or multi-user MIMO (MU-MIMO) mode will use the same common reference signals in all scenarios. Based on its measurements on the common reference signals the UE decides its pre-coding antenna weights (pre-coding matrix) and provides its favorite codebook as feedback information to the base station. However, using only the common reference signal structure may imply disadvantages, e.g. that common reference signals cannot be alone optimal for different scenarios (e.g. high dispersion, line of sight (LoS) channel) and due to the fact that the feedback codebook information (in terms of number of bits) becomes relatively large since the designed codebook should satisfy all the scenarios and all the available antenna solutions.
In addition to common reference signals, dedicated reference signals can be employed to improve channel estimation, weighting verification, etc. For example, in the specification IEEE 802.16 the dedicated pilots are specified as an optional feature that can support the use of open loop pre-coding or closed-loop transmission in which the UE has no knowledge of the pre-coding/beam forming matrix such that not the used weights are signaled on the downlink control channel but rather a pre-coding pilot symbol. Consequently, the UE can use these dedicated pilot symbols to verify that weights are correctly applied to the system. It should be noted that dedicated pilots can be strictly user specific or the same set of dedicated pilots can be assigned to a group of users. The latter approach, obviously, requires less overhead. The specific design of dedicated pilots is not within the scope of this invention but there are different ways to design dedicated pilots, e.g. in US 2007/0025460 and US 2006/0109922. In general there are several benefits of having dedicated pilots. For instance different dedicated pilots can be designed and optimized for different scenarios (high dispersion, LoS channel, etc), channel estimation can be made better and improved estimation of cell orientation can be achieved. Another important advantage is reducing the feedback codebook signaling overheads (reduced number of bits) since scenario-specific codebook can be selected for the scenario under consideration instead of using a bulky code book encompassing all possible scenarios.
The concept of dedicated reference signals for MIMO operations is also applicable in TDD-based E-UTRAN systems.
Either SU-MIMO or MU-MIMO is used at a time within a cell. Switching between the two schemes is desirable to fully benefit from the gains of SU- and MU-MIMO in respective scenarios. The switching can be based on the amount of user traffic, radio conditions, quality of service requirement of users etc.
In the current E-UTRAN system, which is mainly based on the common reference signals, the switching can be either semi-static or fully dynamic. In either case the UEs in the cell are indicated via appropriate signalling. Since all users will use either of the two methods at a time, it is more resource efficient to broadcast the switching-related information to all users in the cell. It is also possible to perform blind switching between MIMO schemes, which implies the advantage that there are no signalling overheads and the switching is very fast; however, the UE is then not aware according to which scheme it is scheduled. Switching between SU-MIMO and MU-MIMO or vice versa is a well known technique used to exploit the benefit of both schemes. However, the assumption in FDD-based E-UTRAN is that the same common reference signals shall be used for both SU-MIMO and MU-MIMO, which is a simple design but not an optimal approach from the perspective of channel and cell orientation estimation. On the other hand, dedicated reference signals could be favoured for TDD-based E-UTRAN systems.
SU-MIMO and MU-MIMO schemes are generally best suitable in different scenarios, e.g. with regard to channel environment and traffic load. Therefore, the optimal pilots required for SU- and MU-MIMO might be different in different scenarios. This also implies that pilots should preferably be associated to the actually used scheme, i.e. specific to MU- and SU-MIMO schemes. In other words, dedicated or MIMO mode specific pilots are needed to achieve an optimum performance and fully exploit the benefits of a particular MIMO scheme.
For example, different schemes are suited in different environments. As shown in FIG. 1, per antenna rate control (PARC) with successive interference cancellation (SIC) is used as the antenna solution for SU-MIMO mode in a low-correlated scenario. On the other hand, a discrete Fourier transform (DFT-) based beam forming solution with orthogonal beam selection is used for MU-MIMO mode in a high correlated scenario. With the DFT-based beam forming solution the antenna weights are taken from discrete Fourier transform (DFT) based matrix and only orthogonal weights are selected to carry the signals of different users in the same resource block. The switching between SU- and MU-MIMO would then be advantageous to optimize the system for the case that the correlation changes in time, e.g. due to changes in the UE position. Similarly, indoor and outdoor environments using half wavelength transmit antennas exhibit different scattering situations where the use of a beam forming scheme is preferable for an outdoor environment but not for indoor environments. The DFT-based pilots can be used for outdoor high-correlated MU-MIMO scenarios whereas distributed FDM pilots can be used for indoor low-correlated SU-MIMO scenarios. It might be sufficient to have only one set of dedicated pilots for a given scheme in a given scenario but when the scenario changes, e.g. due to a change in the system load, a different MIMO mode requiring another set of dedicated pilots can be used. The same set of dedicated pilots can be used for a group of MIMO users.
A set of dedicated pilot as described above differs from the traditional UE-specific pilots in the sense that the latter involves more overhead. Thus, the main advantage of the former approach (i.e. dedicated set of pilots specific to MIMO mode and scenario) is that it ensures good system performance (e.g. in terms of better channel estimation, CQI estimation, cell orientation estimation, etc) compared to common pilots and that it involves lower pilot overhead compared to UE-specific pilots.
In the preceding discussions, it is argued that different dedicated pilot symbols are optimal for SU- and MU-MIMO in different scenarios. Under this assumption, there will be a sharp transition between SU- and MU-MIMO modes when switching between these two modes. This is illustrated by FIG. 2, where at a certain TTI the transmission according to the current MIMO mode seizes and the other MIMO mode starts immediately in the next TTI. The dedicated pilots for SU- and MU-MIMO are respectively associated with certain weighting matrices W1 and W2 characterized by certain phase and amplitude values.
There are two drawbacks with the solution as illustrated by the example of FIG. 2: First, outstanding HARQ retransmissions (if any) at the switching time can either be lost or can be delayed. This is because the same user may not be scheduled immediately when the next MIMO mode starts. Another drawback is that during the first TTI(s) when the new MIMO mode is applied, the UE will report CQI according to the previous MIMO mode. Thus, the channel or cell orientation estimation (forecast) may be based on the wrong dedicated pilots due to the mode switching and at least in the first TTI the decisions for scheduling, power control, and link adaptation will not be based on the appropriate pre-coding matrices leading to a data throughput loss.